Magnesium base alloy



Patented Dec. 2, 1941 MAGNESIUM BASE ALLOY Joseph D. Hanawalt andCharles E. Nelson, Midland, Mich., assignors to The Dow ChemicalCompany, Midland, Mich., a corporation of Michigan No Drawing.

Application March 9, 1940,

Serial No. 323,116

1 Claim.

This invention relates to alloys of magnesium with aluminum, manganese,and zinc. It relates in particular to an alloy containing primarilymagnesium with from 1 to 12 per cent of aluminum, from 0.01 to 0.5 percent of manganese, and from 2 to 4 per cent of zinc.

Among the most desirable commercially available alloys of magnesium fromthe standpoint of its general physical characteristics is the alloy ofcommercial magnesium with from 1 to 12 per cent industry for many yearshas only fair corrosion resistance (.4 to milligrams per squarecentimeter per day) compared to ultra pure magnesium which has acorrosion resistance value of not to exceed 0.2 milligramsper squarecentimeter per day and the commercial quaternary alloy cannot beutilized for many purposes for which it is otherwise well suited.

As is the case of many other pure metals, magnesium itself is not adesirable structural metal and industry has resorted to alloys, amongthe commonest of which is that of magnesium with aluminum, manganese andzinc above described. The commercial alloy of magnesium, aluminum,manganese, and from 2 to 4 per cent of zinc is comparatively easilycorroded in aqueous sodium chloride solution or by brine spray and istherefore unsuited, for example, for use along the seaboard or on boardship where it might be exposed to prevailing atmospheric conditions.'-The comparatively poor corrosion resistance of the commercialmagnesium-aluminum-manganese-zinc alloy is not due to the presence ofthe aluminum or of the manganese or of the zinc but it is due to theexistence ofadditional elements in actually small but stilldisadvantageously high proportions.- Thus, an alloy consisting of verypure magnesium, pure aluminum, pure manganese, and pure zinc andcontaining no other elements has a corrosion resistance at least equalto that of magnesium alone. Such a pure alloy. however, is not asworkable in all respects as the commercial alloy and-further it isimprobable that it could be made generally available economically.

It is an object of the present invention to pro-.

vide an alloy of magnesium with from 1 to 12 per cent of aluminum, from0.01 to 0.5 per cent of manganese, and from 2 to 4 per cent of zincwhich will have a corrosion resistance equal to that of the pure alloyand which, in addition to high corrosion resistance, exhibitsworkability at least comparable with that of the alloy heretoforecommercially available.

The above-mentioned corrosion resistance values of magnesium and itsalloys are determined by means of a now standard procedure known as thealternate immersion method. This method as applied to magnesium alloyscomprises immersing a weighed sample of the material of measured areainto a 3 per cent aqueous sodium chloride solution at room temperaturefor 2' minutes, withdrawing the sample and holding it in air for 1minute, repeating this cycle for a protracted period, and computing theloss in weight of the sample per square centimeter of surface area perday. In the standard tests herein reported, the corrosion resistancevalues are expressed in terms of average weight loss per day per unitarea over a testing period of 112 days.

It has now been found that the foregoing and related objects may beattained and that a. magnesium-aluminum-manganese-zinc alloy can beproduced which will have a corrosion resistance, as measured by thealternate immersion method, at least as good as that of ultra puremagnesium and which exhibits the desired physical properties whichindustry requires by including in the said alloy a very small amount ofat least one of the elements iron, nickel, copper, lead and silicon. Ithas been recognized that the presence of iron in various magnesiumalloys is disadvantageous and that the amount of iron should be kept aslow as possible to minimize corrosion. This is not the whole story,however, and we have found that the mere elimination of iron orreduction of the iron content to a very low value is not suificient toproduce a non-corrosive alloy when other elements are present. It hasbeen found that not only must the amount of iron be minimized but alsothat the amount of numerous other elements must be controlled with'arather narrow range. The presence of at leastone of the elements iron,nickel, copper, lead and silicon is distinctly advantageous from somestandpoints, provided the amount thereof is below the tolerance limit,as will be more fully described hereinafter.

The invention, then, resides in an alloy charactcrized. by a degreeofcorrosion resistance substantially equal to that of pure magnesiumconsisting of from 1 to 12 per cent of aluminum, from-0.01 to 0.5 percent of manganese, from 2 to 4 per cent of zinc and containing at leastone of the elements iron, nickel, copper, lead and silicon in amount notto exceed 0.003 per cent of iron, not to exceed 0.002 per cent ofnickel, not

to exceed 0.5 per cent of copper, and preferably not to exceed 0.7 percent of lead, and not to exceed 1.0 per cent of silicon; the balancebeing magnesium. An alloy composed as above is characterized by having acorrosion rate, in the alternate immersion method in brine, of not toexceed 0.2 milligram per square centimeter per day.

The foregoing tolerance limits for the added elements iron, nickel,copper, lead and silicon apply to the herein claimed composition,whether these elements are present individually or in groups. In thecase of certain magnesium base alloys, it has been determined, and isnow being taught in concurrently filed applications directed to suchsubject matter, that certain of the said added elements exhibit aco-operati-ve corrosion efiect when present together with iron such'thatthe amount of iron and of the co-operative element must be reduced belowthe tolerance limit when both are present. This limitation does notapply to the present compositions which are, therefore, more readilyproducible in quantity and of quality suitable for industrial purposes.

The following table illustrates the critical nature of the tolerancelimit for nickel given in the preceding discussion of corrosion. Theelements named in the tables are the only ones present in significantamounts. The amounts of the respective elements in the alloy beingtested are expressed in per cent by weight of the alloy, the balancebeing magnesium, and the corrosion rates determined by the alternateimmersion method 40 are expressed in milligrams weight loss per squarecentimeter per day based on the average loss over a period of 112 days.

The herein claimed improved alloy in addition to its improved corrosionresistance is at least as hard and strong as the commercial alloy. Thealloy herein claimed differs from that claimed in our co-pendingapplication Serial No. 323,115, filed concurrently herewith, both in thechange in the amount of zinc present, both in the corrosion tolerancelimits, a change in which has been found necessary due to the efiects ofthe larger amount of zinc recited in the present claims. Thus, thepresence of from 2 to 4 per cent of zinc in amagnesium-aluminum-manganese zinc alloy somewhat increases the absolutevalues of the corrosion tolerance limits for the various added elementsherein recited as well as lessening the injurious eflect of the saidadded elements when these are present in amounts greater than thetolerance limits.

We claim:

A magnesium base alloy including from' 1 to 12 per centum of aluminum,from 2.0 to 4.0 per centum of zinc, from 0.01 to 0.5 per centum ofmanganese, and containing nickel in a positive amount not exceeding0.002 per centum, said alloy having a corrosion resistance below about0.2 milligram per square centimeter per day in alternate immersion inthree per centum aqueous sodium chloride.

JOSEPH D. HANAWALT.

CHARLES E. NELSON.

